Fine particles of transition metal oxides, in the several nanometer range, are of particular interest for their potential use in photoelectronics, electromagnetics and as catalyst, catalyst supports and adsorbents. Typically transition metal oxides in this several nanoparticle range are in the form of fine powders and whilst their powdery nature increases the particle surface area they are subject to agglomeration which affects their general performance. Furthermore these fine powders are very hard to recover when used in aqueous systems, thus leading to a potential difficulty in downstream separation.
Various new techniques have been adopted to develop solids of large metal oxide surface area. Antonelli, D. M. and Ying, J. Y., Angew. Chem., Int.Ed. Engl., 1995, 34(18), 2014–2017 and Yang, P. et al, Nature 1998, 396, 152–165 described the formation of TiO2 mesoporous molecular sieves have been synthesised by templated synthesis. Kresge, C. T., et al, Nature, 1992,359,710–712 and Inagaki, S, et al, J. Chem. Soc. Chem. Commun., 1993, 680–682 earlier described the synthesis of mesoporous silica or aluminosilicate. Some approaches, such as starting with metal alkoxides and conducting the synthesis in non-aqueous systems, were employed to overcome serious difficulties in the synthesis, such as those reported in Antonelli, D, M, and Ying, J. Y., Angew Chem Int Ed Engl, 1995, 34(8), 2014–2017. However these processes still did not provided products with suitable pore size and surface areas.
In response to the desire to develop materials with larger pore sizes than zeolites, a class of thermally stable porous materials, pillared layered clays (PILCs) were developed from swellable layered clays, such as smectite, in the late 1970s. Numerous references describe the process, mechanism and properties of PILC's, for example Brindley, G. W. and Semples, R. E., Clay Miner, 1977, 12, 229. It is well understood that when dispersed in water, the layered clays swell because of hydration of the interlamellae cations which act as counterions to balance the negative charges of clay layers, which in turn allows inorganic polycations, the so-called pillar precursors, to be intercalated into the interlayer gallery by cation exchange. During subsequent heating above 400° C., the intercalated polycations are converted to oxide pillars, which prop the clay layers apart. A permanent micropore system is thus formed. Whilst pillaring has become a well-established technique for the synthesis of porous materials the materials produced are limited to microporous solids of a moderate porosity (typical characteristics being, pore volume of 0.15–0.40 cm3/g and BET surface area of 150–450 m2/g), such as those described by Burch, R. Ed, “Pillared clays, Catalysis Today”, Elsevler: New York, 1998, Vol 2 and Mitchell, I. V., Ed. “Pillared layered structures, current tends and applications”, Elsevier Applied Science, London 1990. As the pore size is limited by the formation of pillars, which in turn are limited by the size of the cations being incorporated into the clay structure, it is extremely difficult to obtain large pillar precursors that are identical in size, and result in a catalyst having very high porosity.
Galameau, A., et al., Nature, 1995, 374, 529, reported a successful synthesis of mesoporous solids termed as porous clay heterostructures (PCHs) from layered clays using quaternary ammonium surfactants as template agents. Layered clays were first intercalated with surfactants, tetraethoxide orthosilicate (TEOS), were allowed to hydrolyze and condense, surrounding the intercalated surfactants in the galleries of the clay particles. An open framework of silica formed within the clay layers after removal of the surfactants by heating. Products of this method however have poor porosity characteristics. In the formation of the PCHs the water content present needs to be carefully controlled to ensure that the TEOS is allowed to hydrolyse rendering the product outcome somewhat uncertain.
Suzuki, K. and Mari. T., Appl. Catal., 1990, 63, 181; Suzuki, K. et al., J. Chem. Soc.,Chem. Commum., 1991, 873 and Michot, L. and Pinnavala, T. J., Chem. Mater. 1992, 4 (6), 1433, describe the use of poly(vinylalcohol) (PVA) or alkyl polyether surfactants in the synthesis of aluminium pillared layered clays (Al-PILCs), which resulted in significant changes in the pore structure of the products. The Al-PILC prepared in the presence of PVA however have poor long-range order, and a relatively large pore volume, which mainly arises from mesopores. The PILC catalyst structures of the prior art, the clay layer remains intact while the pillar precursors intercalate into the clay layers by means of ion exchange processes which result in layered clays with a typical diameter of 1–2 μm.
Whilst surfactants have been used to form a templates within catalyst or nanoparticle structures, all the methods of the prior art have suffered one or more limitations, including uncontrollable pore size, limited pore size range, poor porosity characteristics, the clay layers exhibit short range order, and/or their catalytic act is adversely effected when subject to high temperatures. The metal oxide nanoparticles in an exfoliated laponite framework when produced by the method of the invention show surprisingly good porosity characteristics and/or resistance to the effects of high temperatures.